Photocouplers have been used for insulation and separation of signals among input signals and output signals. In recent years there has been a demand for photocouplers that are useful for high-speed analog signal transmission while improving the insulation and separation characteristic. The nonlinearity of photocouplers, particularly the nonlinearity associated with the voltage-current properties of LEDs, becomes a problem in this case. That is, if the photocoupler has a strong nonlinearity, this will result in the distortion of signals during analog signal transmission.
A method of applying negative feedback has been proposed as an example of technology for avoiding this problem (See JP (Kohyo) 11[1999]-509367 and JP (Kokai) 61[1986]-36981). By means of this method, an additional photodiode is positioned close to the light-emitting diode. This photodiode has a structure with which the amount of light emitted from the light-emitting diode is monitored and this is fed back to the operating current of the light-emitting diode.
Nevertheless, in addition to the difficulty of accurate monitoring, it is extremely difficult to design a high-speed circuit with this structure because the amplifier for amplifying the signals requires a band that is at least 10 times the transmission signal bandwidth and nonlinear elements are used. Consequently, this structure cannot be used for high-speed communications.
A method has also been suggested whereby analog signals are not transmitted and instead, analog signals are converted to digital signals and then transmitted, after which they are converted back to analog signals. However, AD/DA converters or additional circuits for modulation-demodulation are necessary in this case, complicating the circuit structure, and high speed operation is still difficult.
Therefore, the present invention seeks to improve the nonlinearity related to current-voltage properties of light-emitting diodes in photocouplers and provide a photocoupler with which relatively high-speed analog signal transmission is possible.